212 METAMORPHOSIS AND 



the lower side. The other three all developed 

 pigment on that side. In one it was first noticed 

 in April 1891, and in the following November the 

 fish was 22 cm. long and had pigmentation over 

 the greater part of the lower side (Plate HI.). Micro- 

 scopically examined, the pigmentation was found to 

 consist of black and orange chromatophores exactly 

 similar to those of the upper side. Some hundreds 

 of young Flounders were reared at the same time 

 under ordinary conditions and none of them de- 

 veloped pigment. 



It is clear, therefore, that exposure of the lower side 

 to light and reduction of the amount of light falling 

 on the upper side (for the tops of the aquaria used 

 were covered with opaque material) does not cause 

 the two sides to behave in the same way in respect of 

 pigment, as they would if the normal condition of the 

 fish was merely due to the difference in the exposure 

 to light of the two sides in the individual lif e. There 

 is a very strong congenital or hereditary tendency to 

 the disappearance of pigment from the lower side, 

 and this is only overcome after long exposure to the 

 light. On the other hand, if the disappearance of the 

 pigment were due to a mutation, were gametogenic 

 and entirely independent of external conditions, 

 there would be no development of pigment after 

 the longest exposure. To prove that an inherited 

 character is an acquired character is quite as good 

 evidence as to show that an acquired character is 

 inherited. The latter kind of evidence is very dif- 

 ficult to get, for the effect of conditions in a single 

 lifetime is but slight, and is not likely to show a 

 perceptible inherited effect. The theory that adap- 

 tations are due to the heredity of the effects of 



